The question of substance in the philosophy of physics has three branches: logical, physical, and epistemological. The first is a problem in pure philosophy: is the notion of “ substance ” in any sense a “ category,” i.e. forced upon us by the general nature either of facts or of knowledge? The second is a question of the interpretation of mathematical physics: is it (a) necessary, or (b) convenient to interpret our formulae in terms of permanent entities with changing states and relations? The third concerns the relation of perception to the physical world.